Q: What is the kAudioUnitProperty_MaximumFramesPerSlice and how do I use it appropriately?A: The kAudioUnitProperty_MaximumFramesPerSlice is used by Audio Units to determine what the maximum number of frames the Audio Unit will be asked to render for any given render call. If you ask an Audio Unit to render for more frames than the value specified by this property, the Audio Unit will return an error and not render any data. Therefore, you need to ensure that value specified by the kAudioUnitProperty_MaximumFramesPerSlice property is always as big as the largest number of frames you would ask to render. IMPORTANT: Your application should always configure and honor this property. It allows the Audio Units to know how big to allocate their internal buffers and perform any additional setup. Additionally, an Audio Unit will not ask its input for more samples than the maximum frames. If more frames are required the Audio Unit will pull multiple times. This can arise in situations, for example, when using the Varispeed Audio Unit where it may be performing playback at 4X speed. Listing 1: Configuring maximum frames
// get the output device's frame count and set any Audio Units to match
UInt32 numFrames;
UInt32 dataSize = sizeof(numFrames);
result = AudioDeviceGetProperty(myDeviceID, 0, false,
kAudioDevicePropertyBufferFrameSize, &dataSize, & numFrames);
if (result == noErr)
{
result = AudioUnitSetProperty(myOutputAU, kAudioUnitProperty_MaximumFramesPerSlice,
kAudioUnitScope_Global, 0, & numFrames, sizeof(numFrames));
result = AudioUnitSetProperty(myMixerAU, kAudioUnitProperty_MaximumFramesPerSlice,
kAudioUnitScope_Global, 0, & numFrames, sizeof(numFrames));
}
If you utilize any sample rate conversions in your Audio Unit chain or graph it is important to take this into account. For example, if the output sample rate of your AUConverter is 44.1kHz and the input is 22.0kHz the AUConverter will scale up its maximum frames by 2X to ensure it will be able to pull the data through correctly. Listing 2: AUConverter scaling
// input sample rate = 22kHz
// output sample rate = 44.1kHz
UInt32 numFrames = 1500;
AudioUnitSetProperty (myAU, kAudioUnitProperty_MaximumFramesPerSlice,
kAudioUnitScope_Global, 0, &numFrames, sizeof(numFrames))
// the AUConverter will scale up
AudioUnitGetProperty(myAU, kAudioUnitProperty_MaximumFramesPerSlice,
kAudioUnitScope_Global, 0, &numFrames, sizeof(numFrames))
// numFrames is now 3000
Note: For maximum memory utilization and performance you may wish to install a property listener on your Audio Unit using AudioUnitAddPropertyListener and listen for sample rate or I/O cycle changes and re-assess the maximum frames. ReferencesBack to Top  Document Revision History| Date | Notes |
|---|
| 2007-07-16 | First Version |
Posted: 2007-07-16
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